It has been found that the conventional single battery automotive electrical systems are often inadequate for engine starting, particularly during cold weather and when the battery has been in use for a long time. Truck engines in particular are difficult to start and the problem is aggravated. It has been found that starting currents as high as 400 amperes can be encountered. It has become common to employ two batteries, particularly in commercial vehicles. One battery operates in the conventional manner and the second battery is connected in series with the regular battery when added starting energy is needed. Obviously a special arrangement must be used to charge the second or start booster battery. This is ordinarily done by connecting it in parallel with the regular battery thereby using the conventional charging mechanism. Since both battery terminals must be switched to change from series to parallel, and, since such switching must accommodate the high starting current circuitry, a heavy duty, double pole relay is usually located near the batteries. The relay is either operated automatically on starting, or from a panel-mounted switch that can be operated when the additional starting energy is deemed necessary. Such a relay is expensive because of the heavy duty current contacts and is subject to failure as is any electromechanical device. Failure propensity of the system is further enhanced by the requirement of making the heavy duty cable connections between the relay and batteries. The large number of schemes for battery switching show that a continuing problem exists in this area.